In one aspect of the present invention, the present invention relates to a dielectric filter for use in mobile communication devices such as an automobile telephone, a portable telephone, etc.
FIG. 1 shows a known dielectric filter. The known dielectric filter is a duplexer type includes a band-elimination filter and a band-pass filter so that the dielectric filter functions also as an antenna coupling circuit device. In FIG. 1, a base substrate 22 is made of dielectric material such as glass-epoxy, Teflon glass or the like and input/output terminals 23, 24 and 25 are formed as electrode films, on an upper surface of the base substrate 22 to be connected to an antenna, a transmitter and a receiver, respectively. An earth electrode 26 is formed as an electrode film on the remaining portion of the upper surface of the base electrode 22 excluding the input/output terminals 23-25 and a lower surface of the base electrode 22. Various electronic components such as seven dielectric coaxial resonators 1-71 are mounted on an upper surface of the earth electrode 26 by soldering, etc. A connection terminal 9 is press fitted into each of holes 8 of the dielectric coaxial resonators 1-7. Furthermore, coupling substrates 13 and 18, spacers 21, a metal cover 27, capacitor substrates, inductors, etc. are mounted on the base substrate 22. A casing of the known dielectric filter is constituted by the metal cover 27 and the base substrate 22.
Because the base substrate 22 is made of glass-epoxy or Teflon glass, the base substrate 22 of the known dielectric filter has the following drawbacks. Namely, the dielectric filter 22 made of glass-epoxy has low heat resistance, a poor temperature coefficient and a poor dielectric loss tangent .delta.. Also, the dielectric filter 22 made of Teflon glass is disadvantageous in that its dielectric constant .epsilon.r is excessively low, its production cost is high because of the need for surface treatment for processing of the through-hole and the material is expensive.
To, obtain the input/output terminals 23-25 and the earth electrode 26 in the known dielectric filter, a copper foil is formed and is subjected, on its surface, to a solder leveller treatment in which air is blown at one side of the base substrate 22 which is dipped in molten solder so as to increase the flatness of the input/output terminals 23-25 and the earth electrode 26.
Thus, in production of the base substrate 22 of the known dielectric filter, because the solder leveller treatment is performed as described above, protuberances of solder are likely to be produced at the time of blowing of air, thereby resulting in nonuniform soldering. In this case, warpage of the base substrate 22 is caused by a difference in shrinkage of solder on the upper and lower surface of the base substrate 22. In addition, the protuberances of solder deteriorate flatness of the base substrate 22.
Furthermore, in the solder leveller treatment, because the base substrate 22, made of for example glass-epoxy, has already been subjected to heat once during its production, stress is generated by the difference in coefficient of thermal expansion of the glass-epoxy, the copper foil and solder, so that warpage or distortion of the base substrate 22 may occur at the time of its delivery to customers. If the customers use the base substrate 22 without noticing such defects of the base substrate 22, the soldering land maybe spaced away from the copper foil whereby soldering cannot be performed because of reflow soldering. Consequently, it is impossible to mount the dielectric coaxially resonators 1-7 on the earth electrode 22.
FIGS. 2(a) to 2(d) show another known dielectric filter 30. A casing of the known dielectric filter 30 is constituted by a base 32 formed by a dielectric substrate and a metal cover 33 for covering an upper surface of the base 32. The base 32 is formed by a flat plate and a plurality of dielectric resonators 34 are mounted on an upper surface of the base 32. Other electronic components such as a chip capacitor, a coiled inductance, etc. are further mounted on the upper surface of the base 32. The cover 33 is mounted on the upper surface of the base 32 to cover these electronic components.
As shown in FIG. 2(c), input/output terminals 35, 36 and 37 which are each formed by an electrode film are provided on a lower surface of the base 32 to be connected to an antenna, a receiver and a transmitter, respectively. An earth electrode 38 which is formed by an electrode film is formed at the remaining portion of the lower surface of the base 32 other than the input/output terminals 35 and 37. The input/output terminals 35-37 and the earth electrode 38 are each connected to an electrode film formed on the upper surface of the base 32 through semicircular through-holes formed at an edge of the base 32.
As shown in FIG. 3, the dielectric filter 30 is a surface mounting type in which the dielectric filter 30 is mounted on an upper surface of a printed circuit board 39 at the side of a producer of communication equipment. When the dielectric filter 30 is secured to the printed circuit board 39 by soldering, the following method has been employed. It should be noted that in FIG. 3, the dielectric filter 30 is depicted to be larger than that of the printed circuit board 39 for the sake of convenience. Namely, as shown in FIG. 3, an earth electrode 40 is formed at a portion of a hot surface of the printed circuit board 39, at area other than patterns to define an area soldered to the earth electrode 38 of the base 32 of the dielectric filter 30. Namely, as shown by hatching of the printed circuit board 39 of FIG. 3, a resist film 41 is formed on the entire surface of the earth electrode 40 except for rectangular portions 42 which form a soldering area for soldering the input/output terminals 35-37 and the earth electrode 38 of the base 32.
As shown in FIG. 4, the following problems arise at the time of mounting the dielectric filter 30 at the side of a producer of communication equipment. FIG. 4 shows soldering between the input/output terminal 36 of the base 32 and a pattern 43 of the hot surface of the printed circuit board 39. As shown in FIG. 4, when the input/output terminal 36 and the pattern 43 have been soldered to each other by solder 44, the solder 44 flows between the dielectric filter 30 and the resist film 41 of the printed circuit board 39 so as to reach the earth electrode 38. As a result, shortcircuiting happens between the earth electrode 38 and the input/output electrode 36 of the base 32 or the pattern 43 of the printed circuit board 39.
When a hot line is provided on the lower surface of the dielectric filter 30 in design of the printed circuit board 39 at the side of a producer of communication equipment, shortcircuiting may take place with an undesirable reduction in the degree of freedom.
Furthermore, in case a portion of the earth electrode 38 of the base 32 acts as an earth electrode 45 and the earth electrode 45 is soldered to the earth electrode 40 of the printed circuit board 39 as shown in FIG. 5, the following problem results. Specifically, solder 44 for soldering the earth electrode 45 proceeds on the surface of the earth electrode 38 of the base 32 of the dielectric filter 30 so as to reach an inner portion of the lower surface of the dielectric filter 30. Hence, shortcircuiting may occur between the earth electrode 38 of the dielectric filter 30 and an electrode 47 formed on a peripheral surface of a through-hole 46 of the printed circuit board 39 at the side of a producer of communication equipment. Solder 44 passes through the through-hole 46 of the printed circuit board 39 and flows to a lower surface of the printed circuit board 39, thereby resulting in shortcircuiting between the earth electrode 38 and components on the lower surface of the printed circuit board 39.
In another aspect of the present invention, the present invention relates to a method of soldering electronic components and more particularly, to a method of preventing a so-called resorption phenomenon in which metals of the electronic components dissolve into solder at the time of soldering.
Conventionally, an alloy of Pb and Sn has been employed as solder for soldering electronic components. When soldering is performed by using the known solder alloy referred to above, it is known that resorption happens. Thus, metals used for the electronic components dissolve into molten solder. Almost all kinds of metals dissolve into solder.
Meanwhile, the metals which have dissolved into solder by resorption are deposited as a compound with Pb or Sn. Because the deposited compound is quite fragile, mechanical strength of solder containing the compound is extremely small. Furthermore, because the compound is likely to be formed at a boundary between base metal and solder, soldered portions become weak and unable to withstand against vibrations or bending causing the soldered portions to be peeled from the boundary face. Therefore, the electronic component in which soldering is performed in the known method has such drawbacks that mechanical strength of the soldered portions is reduced and its electrical characteristics deteriorate.